Move caller_filter to `rspec/caller_filter`.

- The caller_filter is generic for all of RSpec, not just
  rspec-mocks specific.
- It's also at `rspec/caller_filter` in core (and will be
  in rspec-expectations soon). The idea is that any lib can
  require `rspec/caller_filter` and get this.
- Add spec to verify the file stays up to date.
Remove the benchmark -- it's been added to rspec-core.
Fix load path search.

It worked fine before with a local bundler path, but not
a :git path -- the :git path includes the SHA in the
directory.
Merge pull request #411 from rspec/move_caller_filter

Move caller_filter to `rspec/caller_filter`.